The present invention relates to a holographic reconstruction system with an enlarged visibility region, and to an according method. The holographic reconstruction system comprises light source means for providing substantially coherent light, reconstruction means for the holographic reconstruction of a scene and for generating a visibility region from which an observer can watch the holographically reconstructed scene, and deflection means for positioning the visibility region.
In a holographic reconstruction system, sufficiently coherent light is modulated by spatial light modulator means (SLM), e.g. a liquid crystal display (LCD). A diffractive structure, the hologram or a sequence of holograms, is encoded on the SLM. Object light points are generated through interference of the light which is modulated with holograms in the SLM. The entirety of those object light points form the three-dimensional reconstruction of an object or scene. The light of all object light points propagates in the form of a light wave front, so that one or multiple observers can watch those object light points from an eye position as a three-dimensional scene. For the observer, the light appears not to come from the SLM, but from the three-dimensional object reconstruction, i.e. from multiple depth planes. The observer focuses his eyes on the object reconstruction with its multiple depth planes. The eyes can only see the light which is diffracted by the SLM, but not the light which is transmitted directly. When watching a holographic display, an observer thus ideally has the same impression as if they watched a real object. This means that in contrast to a stereoscopic representation, a holographic reconstruction realises an object substitute, which is why the problems known in conjunction with stereoscopy, such as fatigue of the eyes and headache, do not occur, because there is generally no difference between watching a real scene and a holographically reconstructed scene.
Known holographic reconstruction systems, for example as disclosed by the applicant in the international patent application WO2004/044659, are substantially based on the following general principle: A wave front which is spatially modulated with holographic information reconstructs the three-dimensional scene in a reconstruction space which is positioned in front of one or both eyes of one or multiple observers. The holograms can also be encoded such that the object light points do not only appear in front of, but also on and behind the display screen. The reconstruction space stretches from the exit surface of a display screen, through which the modulated wave fronts leaves the reconstruction system, to a visibility region. The visibility region has a finite extent in one plane, for example corresponding to the size of an eye or eye pupil. If at least one eye of an observer is situated in the visibility region, the observer will be able to watch the holographically reconstructed scene.
The size of the visibility region depends on the focal length of the holographic reconstruction system, the wavelength of the used light and the pixel pitch of the spatial light modulator for encoding the scene to be holographically reconstructed. The larger the desired visibility region the higher must be the resolution of the SLM used. In order to get a large visibility region, the SLM must have very small pixel apertures which cause great diffraction angles, i.e. the SLM must have a small pixel pitch and, consequently, a large number of pixels.
In order to reduce the necessary resolution of the SLM, the size of the visibility region can for example be diminished to the size of an eye pupil. However, this may lead to problems with the visibility of the three-dimensional reconstruction, if the observer eye is only partly situated inside the visibility region. Already a slight movement of the observer may cause effects such as disappearance of visibility, vignetting or distortion of the spatial frequency spectrum. Moreover, the borders of the reconstruction space are difficult to find for an observer whose eyes are situated outside the visibility region. It is therefore necessary for the position of the visibility region to be adapted to the new eye position if an observer moves.
Because in a small visibility region the observer can see the holographic reconstruction with one eye only, a second wave front, which is directed at the other eye, must provide a second reconstruction which differs in parallax. Because both reconstruction spaces must have the same base on the display screen in order to ensure perception of the two reconstruction spaces free from optical errors, their respective wave fronts are spatially or temporally interleaved with the help of known autostereoscopic means. Spatial frequency filters and focusing means prevent optical cross-talking between the wave fronts. If the reconstruction system is additionally meant to allow multiple observers to watch different reconstructions simultaneously, additional wave fronts will be required, typically two for each observer. These additional waves can be generated either in a space-division or in a time-division multiplex process.
In order to maintain a certain clarity, the description below relates mainly to the alignment of a single wave front of the holographic system. The reconstruction system can realise further wave fronts in analogy to the first one, if required. It appears to those skilled in the art that the idea of this invention can be applied as often as necessary for this, depending on the actual number of wave fronts. When doing so, functional elements of the invention can preferably be used commonly for multiple wave fronts.
Known systems comprise an eye finder and a deflection unit for example a scanner mirror. The eye position is detected by the eye finder. The required angular position of the deflection unit is found based on that eye position, and the deflection unit is controlled accordingly in order to match the position of the visibility region to the eye position. At the controlled position, the deflection unit must rest for a moment so that the hologram can be reconstructed. Then, the next eye position is detected and so on. This causes the deflection unit to move intermittently, which is difficult to be realised using conventional means, in particular at high frequencies, e.g. higher than 20 Hz.
With a small visibility region, it is further required that the eye finder detects the eye position with a very high accuracy. For example, if the size of the visibility region is between 5 to 10 mm, the eye finder should detect the eye position with a maximum error of about 1 mm. Again, this is difficult to be realised using conventional means.